# A three-input NAND gate is to be used as an inverter. Which one of the following measures will achieve better results?

1. The two inputs not used are kept open
2. The two inputs not used are connected to ground (0 level)
3. The two inputs not used are connected to logic supply (1 level)
4. None of the above

I get that we can either connect them to ground or make them both high, and the output will remain the same. But connecting to high seems to be a better alternative. Why is it so?

• Do the truth table. Oct 12, 2022 at 13:39
• Can they, though? [be connected to ground] What about other options, like tying inputs in common -- I take it this is a multiple-choice with only these shown? Oct 12, 2022 at 13:43
• None of the above would be my answer by a short neck. Oct 12, 2022 at 13:43
• @Andyaka I think the words BETTER results in the question makes one of the first three choices acceptable. Oct 12, 2022 at 13:57
• Only one option will deliver the expected result. There’s no option for connecting all the inputs together which would be a valid solution. As the ratman suggests, look at the truth table - it is the source of truth! Oct 12, 2022 at 14:40

## 1 Answer

You don't specify the technology. Answers for TTL and CMOS are slightly different, and there are quite a few others as well. You also don't specify an objective for what you call "better".

Short of the technology in question, the only facet of the answer you can specify is that you need to connect unused inputs either to something interpreted as logical 1 (or the output will get fixed at logical 1 irrespective of the input) or to the input to be inverted.

Which kind of input that is interpreted as logic 1 will be best depends on the technology. Some will take VCC directly, some may warrant a resistor for protection against transient behavior. While I wouldn't fancy it, TTL logic would not mind open pins a lot, either.

• Lucky it is just a multiple choice question! Oct 12, 2022 at 15:15